In the course of ongoing environmental legislation, there is a growing need for sensors which are able to assist in determining even very small quantities of pollutants reliably. A large role is played here in particular by measuring methods which make it possible to determine the presence of gaseous pollutants in the ppm range. In this connection, however, it is particularly challenging to determine the content of nitrogen oxides in combustion exhaust gases, due to the high proportion of oxygen which is also present. This has significance for possible utilization as OBD sensors to fulfill coming exhaust gas legislation, because in this context nitrogen oxide limits, for example, will be prescribed which are below the resolution limit of currently available solid electrolyte gas sensors.
In order to make proper allowance for these problems, a storage device for a sensor element for qualitative or quantitative determination in particular of nitrogen oxides in a gas mixture is described, for example, in German Patent Application No. DE 10 2007 050 119 A. The storage device described therein, or the sensor element containing such a storage device, is based on the idea of accumulating gas components to be determined which are present in only a low concentration in a gas mixture being analyzed, over a certain time period. To that end, a storage device which is resistant to contamination and temperature changes is provided within the sensor element.
The determination of the gas component to be detected takes place discontinuously, the gas component to be determined first being stored in a storage medium during a collecting phase and the storage state of this storage medium being detected. In a subsequent regeneration phase, the stored gas component which is to be detected is then removed again from the storage medium, for example by raising the temperature. Alternatively, the gas component to be detected which is released during the regeneration phase may be detected and its concentration ascertained.
In addition, a sensor for determining gas components in gas mixtures is described in European Patent No. EP 1 739 416 A2, which may be used among other things for determining nitrogen oxides. The described gas sensor includes a plurality of electrochemical cells and a chamber having limited inflow, which contains a reference gas atmosphere as the reference point for determining the gas component which is to be detected. In this chamber having limited inflow there are internal electrodes of electrochemical cells, with the aid of which the content of gas components which are to be detected may be determined by voltametric means.